A Fan Powered Variable Air Volume (FPVAV) terminal unit is a type of VAV box that incorporates a small, independently powered fan to supplement or recirculate airflow within a conditioned zone. Unlike standard VAV terminals that rely solely on primary air from a central air handling unit, FPVAV units mix primary conditioned air with return (plenum) air to maintain consistent temperature and airflow. This design allows the system to reduce primary air volume while still delivering adequate ventilation and comfort at the zone level.
Technical Details and Specifications
FPVAV units are available in two primary configurations: series and parallel. In a series FPVAV, the integral fan runs continuously, drawing both primary and return air through the unit at all times. This ensures constant airflow to the space regardless of the primary air damper position. In a parallel FPVAV, the fan operates only when the primary air volume drops below a set threshold, typically during heating mode, pulling return air from the ceiling plenum to supplement reduced primary airflow.
- Fan Motor Size: Typically ranges from 1/8 HP to 1 HP, selected based on the required airflow volume and available static pressure.
- Primary Air Reduction: FPVAV units can reduce primary air volume to as low as 20-30% of peak cooling demand, decreasing the load on the central air handling system.
- Airflow Mixing Ratio (Parallel Units): Return air to primary air ratios commonly range from 2:1 to 4:1, depending on unit sizing and application.
- Heating Capacity: Electric or hot water reheat coils typically range from 1 kW to 10 kW to meet zone heating loads.
- Control Signals: Units typically accept 0-10 VDC signals for damper modulation and fan speed control, integrating with building automation systems (BAS).
Common Applications
Fan powered VAV terminals are widely used in commercial buildings that require precise zone-level temperature control combined with energy-efficient operation. Typical installations include office buildings, schools, hospitals, and retail spaces. They are especially valuable in interior zones with variable occupancy and in perimeter zones where supplemental heating is needed during cold weather. Series units are often preferred in applications where constant airflow and ventilation are critical, while parallel units are selected where energy savings from intermittent fan operation are prioritized.
Relevant Standards and Codes
FPVAV units must comply with ASHRAE Standard 90.1 requirements for fan energy and minimum ventilation airflow. ASHRAE Standard 62.1 governs outdoor air ventilation rates, which directly affects how primary air minimums are set for these terminals. UL 1995 covers the safety certification of heating and cooling equipment, including fan powered terminals. Local mechanical codes may also specify requirements for plenum-rated construction and electrical connections.
Practical Significance
By reducing the volume of primary air that the central system must condition and deliver, FPVAV units lower overall fan energy consumption and allow for smaller ductwork in some cases. The ability to recirculate warm plenum air during heating mode recovers energy that would otherwise be wasted. For building operators, these units provide fine-tuned zone control that improves occupant comfort and reduces complaints, making them a reliable choice for multi-zone commercial HVAC systems.